Converters presently used to convert sulphur dioxide gas to sulphur trioxide gas are typically large cylindrical vessels comprising a shell containing a number of catalyst beds disposed one above the other. The processed gases pass through the catalyst beds in several, optionally desired sequences and are cooled between beds both to recover the heat generated in each bed and to assist in the kinetics and equilibrium of the reaction. Each bed is separated from other beds by division plates or membranes.
Classically, in sulfuric acid manufacturing plants, converters were fabricated from carbon steel, cast iron, and brick, when these materials were the only ones available. Carbon steel was used for the shell and cast iron posts, beams, and plates or sections were assembled inside the converter to support the many beds of catalyst.
In an alternative design, brick structures were erected for the same purpose, with steel being used for the external shell. At the time such converters were designed, manufacturing plants were small in capacity and gas strengths were low, which resulted in modest gas temperatures. In addition, platinum catalyst was used which tended to required operations at temperatures below those at which present-day catalysts operate. Vanadium catalyst, much larger plant capacities with higher internal pressures and much higher gas strengths have drastically increased the mechanical loads on such converter shells, while at the same time the conventional carbon steel becomes hotter and, hence, much weaker. Distortion of the vessel and, thus, leakage are, therefore, more common. However, such converters are well-known in the industry and a discussion of their features can be found in many references to sulfuric acid manufacture.
It has been known for many years that stainless steel is much stronger than carbon steel at temperatures now found in sulfuric acid plant converters as well as possessing resistance to scaling caused by hot sulphur dioxide containing gases. In the last ten to fifteen years a number of stainless steel converters have been built and are in operation.
One such converter type is described in McFarland U.S. Pat. No. 4,335,076, wherein is used a vertical, cylindrical, stainless steel shell having horizontal catalyst beds, one above the other, with the catalyst beds being supported in part from the shell. In a preferred embodiment, the beds and divider plates are supported from a hollow cylindrical core tube, as well as the shell, with the catalyst being contained in the annulus between the core tube and the shell. The vertical cylindrical core is co-axially located and extends the full height of the shell from the bottom of the converter to the top.
In some commercial applications of U.S. Pat. No. 4,335,076, the core tube contains a heat exchanger which cools gases between the catalyst beds and, in at least one case, two pieces of heat transfer apparatus are contained within the core tube. Gas is fed to catalyst beds either from an annulus around the bed in the case of the first bed, with radially, inward flow to the space above the catalyst or, in the other beds, horizontally, radially outward from the core of the vessel, with the gas entering the core from the top of the vessel.
This approach has proven reasonably successful but lacks versatility in many ways. Use of the large diameter core requires a larger-sized converter if the same bed cross-sectional area as is typically used in a sulphuric acid plant, is desired, which results in a larger and more expensive vessel. In addition, axial entry of gases as is taught in U.S. Pat. No. 4,335,076, is often not convenient for the designer of sulphuric acid plant systems because it requires more ducting. Inclusion of heat transfer equipment in the converter as is disclosed in U.S. Pat. No. 4,335,076, is superficially more attractive, but restricts process design freedom. With internal equipment, it is also no longer practically possible to parallel some of the heat exchange steps and provision of effective by-passes around the internal equipment is more difficult. It also complicates preheating of the plant and cooling for catalyst screening. Further, the converters of U.S. Pat. No. 4,335,076 do not offer an effective method for introducing gases to catalyst beds through the vertical wall of the vessel.